The wireless IC tag (including a wireless IC card, and hereinafter generally referred to as “IC tag”) generally has an antenna circuit formed on a surface of an insulative substrate such as a resin film. An IC chip is mounted on the substrate carrying the antenna circuit, and connected to the antenna circuit.
Because of carrying the IC chip, the storage capacity of the IC tag is far beyond that of a barcode or an ID tag, and also accepts free writing of additional information. In particular, a flat type IC tag is widely utilized because of ease in transportation and attachment to other devices.
Now, a less expensive IC tag, which yet offers the excellent communication characteristics, is being eagerly desired, for further expanding the utilization of the IC tag.
Regarding such IC tag, for example, a patented document 1 discloses an IC tag that has a circuit pattern formed of a metal such as copper or silver, or a conductive paste.
This document also discloses methods of forming the circuit pattern, including winding a coated wire in a coil shape, and forming a coil pattern by an etching process over a conductive metal such as copper laminated on the substrate.
Regarding the description in the patented document 1, however, the metal such as copper or silver is expensive, which impedes reducing the material cost. Besides, the circuit pattern formed of the conductive paste is inferior in conductivity to a coil pattern formed of a metal thin film made by vapor deposition or plating, and hence consumes larger power but offers a shorter communication range, in other words the communication characteristic is degraded. To overcome such drawback it is necessary to increase the line width of the coil pattern, which incurs not only an increase in material cost due to the expansion of the line width, but also an additional disadvantage that a floating capacitance emerges.
Also, winding the coated wire in a coil shape requires a troublesome work, which impedes reducing the manufacturing cost.
Further, forming a copper thin layer all over the surface of the insulative resin and performing the etching process to thereby form the coil pattern constitutes factors that inhibit the cost reduction, because such method requires complicated works not only for the formation process but also for the incidental works such as a preliminary process and disposal of wastes, the manufacturing equipment has to be complicated and large-scaled and the applicable substrates are limited.
It might be an option to employ aluminum or silver instead of copper, however those metals, including copper, all have a high melting point, which makes those metals difficult to be processed.
In addition, those metals each have particular drawbacks, such that copper is susceptible to oxidation and hence unsuitable for a heat treatment; aluminum has low conductivity; and silver has poor spreadability and is hence difficult to be formed into a foil.
Referring now to FIG. 8, a conventional IC tag will be described. FIG. 8 schematically depict a structure of the conventional IC tag, in which FIG. 8(a) is a plane view thereof, and 8(b) a fragmentary cross-sectional view taken along a line A-A′ of FIG. 8(a).
In FIG. 8, the reference numeral 301 designates the IC tag, and 302 designates an electrically insulative substrate. 303 to 306 designate those composing antenna circuits, among which 304 and 306 designate the respective end portions of the antenna circuits. 307 designates an IC chip, and two electrodes of the IC chip 307 (not shown) are each connected via the antenna circuits 303 and 305 respectively. From the two end portions 304 and 306 of the antenna circuit formed on one surface of the substrate 302, through holes 311, 312 are respectively provided so as to penetrate to the other surface of the substrate, and the through holes 311, 312 are, for example, filled with solder, after which a jumper wire 310 made of a conductive wire is fixed and connected to the through holes, on the other surface of the substrate 302. The solder may be substituted with a conductor having a fastening function such as an eyelet, for fixing and connecting the jumper wire.
Meanwhile, the resonant frequency of the foregoing IC tag is determined from the static capacitance specific to the IC chip 307, and the inductance obtained from the length of the antenna circuits 303 to 306 connecting the through holes 311, 312. Accordingly, methods for adjusting the resonant frequency of the IC tag so far proposed include providing a capacitor and a resistor in the antenna circuit, to thereby add the static capacitance (patented document 2), and occasionally connecting and disconnecting a plurality of antenna circuits so as to change the length of the antenna circuit (patented document 3).
The proposal according to the patented document 2, however, incurs an increase in number of parts and hence in manufacturing cost, since the capacitor for adjustment and the resistor for adjustment are provided in the antenna circuit. Besides, the IC tag thus configured may shorten the communication range.
Regarding the patented document 3, the length of the antenna circuit is adjusted through forming a plurality of electrically connected antenna coils on the surface of the insulative substrate and cutting off the connection of an unnecessary antenna circuit if needed, and therefore the structure of the antenna circuit becomes very complicated and the manufacturing cost is increased. Also, the IC tag thus configured may be larger in size.
Thus, the conventional IC tag cannot be evaluated as sufficiently inexpensive yet having excellent communication characteristics, and it is impossible to easily and inexpensively adjust the resonant frequency.    Patented document 1: JP-A No. 2006-304184    Patented document 2: JP-A No. 2001-010264    Patented document 3: JP-A No. 2001-251115